This invention relates to a rotary head for a drilling machine, for example a drilling machine for use in harbor construction, the construction of foundations for bridges or highways or railways, tunnel drilling and the like.
Most previously proposed rotary heads used in the reverse-circulating drilling method have a flushing outlet at the upper end thereof and a connection device, for example a screw-threaded end, for connection to a drilling tool at the lower end. Therefore it is necessary to attach a one inch (2.4 cm) air pipe fitting outside the drill pipe or tools. In addition, it is necessary to make sure that every drill rod is in line with the air pipe. Therefore a considerable amount of time is required to assemble and disassemble parts such as the drill string and tools for use. Thus, such a rotary head has a very low efficiency or productivity.
Recently, over-drilling methods have been proposed in which a drill rod and a drill casing can be rotated simultaneously to drill into the ground to be bored. Although, such an arrangement has increased efficiency, the drill rod cannot be seen from the outside and therefore, when the drill casing is unscrewed, the drill rod may easily become unscrewed without being noticed and may therefore drop into the bore.
Furthermore, previously proposed rotary heads normally have a rotation speed below 50 revolution per minute (rpm), the highest speed being approximately 100 rpm, and the efficiency of drilling bits such as roller bits or diamond bits cannot be maximized at such low speeds so that such rotary heads have a poor uneconomic performance.